Apple has released ten generations of iPhone models, each accompanied by one of the ten major releases of the iOS operating system. The original 1st-generation iPhone was a GSM phone and established design precedents, such as a button placement that has persisted throughout all releases and a screen size maintained for the next four iterations. The iPhone 3G added 3G network support, and was followed by the 3GS with improved hardware, the 4 with a metal chassis, higher display resolution and front-facing camera, and the 4S with improved hardware and the voice assistant Siri. The iPhone 5 featured a taller, 4-inch display and Apple's newly introduced Lightning connector. In 2013, Apple released the 5S with improved hardware and a fingerprint reader, and the lower-cost 5C, a version of the 5 with colored plastic casings instead of metal. They were followed by the larger iPhone 6, with models featuring 4.7 and 5.5-inch displays. The iPhone 6S was introduced the following year, which featured hardware upgrades and support for pressure-sensitive touch inputs, as well as the SE—which featured hardware from the 6S but the smaller form factor of the 5S. In 2016, Apple unveiled the iPhone 7 and 7 Plus, which add water resistance, improved system and graphics performance, a new rear dual-camera setup on the Plus model, and new color options, while removing the 3.5 mm headphone jack found on previous models.